A train braking device having an air brake controller is configured such that a pressure control signal is generated based on a brake command and a traveling velocity of a train, a relay valve outputs a brake cylinder pressure corresponding to the pressure control signal, and the brake cylinder pressure acts on brake shoes so as to obtain a predetermined brake force. The brake force is calculated by a product of the brake cylinder pressure and a friction coefficient of the brake shoe. Meanwhile, the friction coefficient changes in a region where the traveling velocity of a train is low and also changes according to a period of use of the brake shoe. To obtain a stable brake force, a train braking device that can control the brake cylinder pressure appropriately according to the change in the friction coefficient is required.
A train braking device described in Patent Document 1 mentioned below is configured to calculate a friction coefficient for each brake notch and then to calculate a brake force. Further, a brake force according to a change in the friction coefficient at low velocity is calculated by using a predetermined friction coefficient pattern.    Patent Document 1: Japanese Patent Application Laid-open No. H11-235972